The slide of a press is generally driven such that it is lowered at low speed conformable to processing conditions within the zone of a forming phase and moved at high speed within other zones than the formation zone, whereby the cycle time of the press is decreased to achieve improved productivity. To obtain such slide motion, there has been conventionally used a link drive press in which the slide is driven by the main motor through a complicated link mechanism. The link mechanism of the link drive press is designed to make the speed of the slide within the formation zone alone (formation speed) slow and to make the speed of the slide within other zones (e.g., lifting phase) than the formation zone a bit faster. The speed difference of the link drive press is up to about 30% of the speed difference of the crank press.
Needless to say, improved productivity is one of the most important themes (demands) for press work carried out by the users of press machines. As an attempt to achieve improved productivity, the rotational speed of the slide drive shaft is increased in mechanical presses such as the above-described link drive press. However, increasing of the rotational speed of the drive shaft causes a proportional increase in the slide speed (i.e., touching speed at which the press touches the workpiece) within the formation zone, which brings about the problem that the resulting speed does not meet the desirable forming conditions. In addition, noise occurring when the press touches the workpiece increases. In view of this, the rotational speed of the slide drive shaft cannot be increased so much, and therefore there is a limit to improving productivity.
As a means for solving the above problem, driving of the link mechanism with an electric servo motor is conceivable, but this also reveals such a drawback that a large-sized electric servo motor having larger output torque becomes necessary for generating a pressing force substantially equivalent to a sum of the output torque of the conventional main motor and the accumulating energy of the flywheel. Use of a large-sized servo motor leads to an increase in the cost and size of the overall press machine. Furthermore, in cases where a press that has long been in service is modified (i.e., retrofitting), large-scaled reconstruction becomes necessary to replace the conventional main motor with a large-sized electric servo motor, causing problems such as a prolonged reconstruction period and increased reconstruction cost.
The present invention has been directed to overcoming the above shortcomings, and a primary object of the invention is therefore to provide a press drive unit and a press drive method which improve the cycle time of the press to achieve increased productivity, provide improved product quality and enable use of a small-sized, inexpensive press.